Method for recycling waste lithium iron phosphate

Abstract

A method for recycling waste lithium iron phosphate, belonging to the technical field of material recycling. The method specifically separately reprocesses the precipitate and leachate obtained by separating and leaching waste lithium iron phosphate powder, using membrane concentration to highly enrich the lithium ions in the leachate and fully utilizing the filtrate generated during the treatment process, and ultimately can simultaneously and efficiently recover and obtain lithium salt and iron phosphate products. The entire method can achieve a lithium recovery rate of 95% or greater for waste lithium iron phosphate, the maximum concentration of impurity elements in the obtained iron phosphate not exceeding 5 ppm, and the quality of the iron phosphate product being relatively high.

Description

A method for recycling waste lithium iron phosphate Technical Field

[0001] This article relates to the field of material recycling technology, and in particular to a method for recycling waste lithium iron phosphate. Background Art

[0002] Lithium iron phosphate-based lithium batteries are a type of power battery with high electrochemical properties and high safety. Generally, the positive electrode plates containing lithium iron phosphate in such batteries are recycled in a special recycling system after being deactivated and scrapped. The most common method for recycling waste lithium iron phosphate is wet recycling, which is to leach out all the metal ions (mainly lithium ions) in the lithium iron phosphate, and then selectively recover the valuable metal ions by removing impurities and step-by-step precipitation. However, the recovery rate of traditional wet recycling is low. The main reason is that some metal ions are not recovered in the waste residue or leachate obtained by precipitation during the treatment process. If the recovery rate of these metal ions is to be improved, it is necessary to spend a lot of money to upgrade the recycling equipment such as the evaporation concentration system (for example, upgrading to an MVR system), or it is necessary to sacrifice some recyclable products, and it is impossible to achieve low-cost and efficient integrated recycling of waste lithium iron phosphate.

[0003] Summary of the Invention

[0004] The purpose of this article is to overcome the shortcomings of the above-mentioned existing technologies and provide a method for recycling waste lithium iron phosphate. This method specifically reprocesses the precipitate and leachate obtained by separating and leaching waste lithium iron phosphate powder, and uses membrane concentration to highly enrich the lithium ions in the leachate and fully utilizes the filtrate generated during the treatment process. Ultimately, lithium salts and iron phosphate products can be efficiently recovered and obtained at the same time.

[0005] To achieve the above objectives, the technical solutions adopted in this paper are:

[0006] A method for recycling waste lithium iron phosphate comprises the following steps:

[0007] Placing the electrode containing waste lithium iron phosphate into an acid solution containing a first oxidant for leaching to obtain a leachate and leach residue;

[0008] The leachate is adjusted to pH 8-9, impurities are precipitated and filtered to obtain a primary filtrate and a primary filter residue;

[0009] The primary filtrate is concentrated by a membrane system and then precipitated by adding a precipitant to obtain lithium salt and a secondary filtrate;

[0010] The leached residue is reacted with an acid solution under an inert atmosphere, and then a reducing agent and an additive are added for mixed reaction, filtered, and a second oxidant is added to the obtained mixed solution for reaction, filtered, and ferric phosphate and a tertiary filtrate are obtained; the reducing agent is elemental iron, and the additive is at least one of ferric phosphate dihydrate, ferrous phosphate octahydrate, titanium hydrogen phosphate monohydrate, aluminum hydroxide, titanium dioxide, and titanium trioxide; the mass ratio of the leached residue, the reducing agent, and the additive is 100:(20-65):(50-200);

[0011] The secondary filtrate and the tertiary filtrate are mixed and the pH is adjusted to 9-10, filtered, and then the pH is adjusted to 4-6 and filtered again; the obtained filtrate is subjected to fine filtration, ultrafiltration and reverse osmosis treatment to obtain purified water and a concentrate with a pH of 2-4, the concentrate is placed in the primary filter residue and leached, filtered, and the fourth filtrate is treated according to the primary filtrate treatment steps.

[0012] In the method for recycling waste lithium iron phosphate described herein, waste lithium iron phosphate is first selectively extracted with an oxidant and an acid solution to obtain a leachate containing lithium ions and a leachate residue containing iron, phosphorus, and other electrode materials such as graphite. The leachate is then pH-adjusted to allow some impurity ions, such as iron and aluminum ions, to precipitate as hydroxides or oxides. The resulting primary filtrate is then concentrated using a membrane concentration process. Compared to existing MVR system components, this process prevents crystallization of lithium salts in the filtrate, further improves the purity of the resulting concentrate, and reduces energy consumption. After concentration, a precipitant is added to obtain high-purity lithium salts. Simultaneously, iron powder is added to the leachate residue containing iron and phosphorus as a reducing agent, and a specific type of additive is introduced. The two substances react synergistically, inducing the precipitation of impurities such as titanium in the filter residue and reducing iron loss, effectively removing impurities. The product after impurity removal is oxidized to obtain high-purity iron phosphate. Finally, in order to achieve a high lithium ion recovery rate of waste lithium iron phosphate, the technical solution described in this article also recovers the secondary filtrate and tertiary filtrate generated in the above process and converts them into a concentrate. The concentrate is in liquid state and mainly contains lithium ions that have not been recovered in the previous process. At the same time, this concentrate is acidic and can be used for leaching of the primary filter residue, so that the lithium ions in the filter residue and the lithium ions in the concentrate are both enriched in the filtrate obtained by leaching. Further membrane concentration and other process treatments are carried out according to the treatment steps of the primary filtrate to achieve lithium ion recovery of the entire recyclable material.

[0013] In one embodiment, the first oxidant is at least one of oxygen, hydrogen peroxide, and sodium persulfate.

[0014] In one embodiment, the acid solution is an aqueous solution of at least one of sulfuric acid and hydrochloric acid.

[0015] In one embodiment, the pH of the leachate after pH adjustment is 8.5.

[0016] Within the pH range, the leachate can fully precipitate impurities such as ferric ions, ferrous ions, and aluminum ions, while also preventing the precipitation of some lithium ions and ultimate lithium loss due to excessively high pH.

[0017] In one embodiment, the membrane system includes a nanofiltration membrane system and a reverse osmosis membrane system in sequence.

[0018] Compared with the traditional MVR system, the membrane system described in this article first uses a nanofiltration membrane system to separate metal impurity ions such as calcium, magnesium, and aluminum in the primary filtrate, and then uses a reverse osmosis membrane system to enrich the lithium ions in the solution.

[0019] Furthermore, the rate at which the primary filtrate enters the membrane concentration system is 380 to 420 L / h, the pH is 2 to 4, and the pressure is 1 to 1.5 MPa.

[0020] Furthermore, the lithium ion concentration in the solution of the primary filtrate after concentration by the membrane system is 20 to 25 g / L.

[0021] In one embodiment, the precipitant is at least one of sodium carbonate and sodium phosphate dodecahydrate, the temperature during the precipitation is 80-100° C., and the time is 2-4 hours.

[0022] In one embodiment, the reaction time of the leaching residue with the reducing agent and the additive is 3 to 6 hours.

[0023] Under the synergistic effect of the iron element and the additives in the ratio described in this article, the reaction is carried out for 3 to 6 hours, and the impurity removal rate of the leaching residue described in this article can reach more than 90%, so the prepared regenerated iron phosphate has a high purity.

[0024] In one embodiment, the second oxidant is one of oxygen and hydrogen peroxide.

[0025] In one embodiment, the second oxidant is hydrogen peroxide, and the addition rate of the hydrogen peroxide is 0.7-1.1 mL / min.

[0026] At the aforementioned addition rate, the introduction of hydrogen peroxide does not cause the growth of precipitated grains to be too fast or too slow, and the ultimately obtained iron phosphate has a good crystal form and morphology.

[0027] In one embodiment, the second oxidant is oxygen, and the pressure of the introduced oxygen is 1.2-1.5 MPa.

[0028] Similar to hydrogen peroxide, under the above-mentioned injection pressure, the crystal form and morphology of the prepared iron phosphate are better.

[0029] In one embodiment, the temperature of the mixed solution during the reaction with the second oxidant is ≥80°C.

[0030] When the temperature of the ferric phosphate formation reaction is controlled at 80°C or above, the nucleation rate of the grains can be effectively controlled within a lower range, thereby maintaining the particle size of the prepared ferric phosphate at a more appropriate size.

[0031] In one embodiment, the unit dosage of the concentrate for leaching the filter residue once is 3-7 mL / g.

[0032] Furthermore, the concentration of iron in the concentrate is less than 0.001 g / L, the concentration of phosphorus is less than 0.001 g / L, the concentration of copper is less than 0.0001 g / L, the concentration of aluminum is less than 0.0001 g / L, and the concentration of lithium is 4-5 g / L.

[0033] Furthermore, the secondary filtrate and the tertiary filtrate are mixed and the pH is adjusted to 9-10, filtered, and then the pH is adjusted to 4-6 and filtered again; the obtained filtrate is sequentially passed through a microporous filter, a plate heat exchanger, and an ultrafiltration device to complete fine filtration and ultrafiltration treatment, and then placed in an ultrafiltration water production tank; the water in the ultrafiltration water production tank is subjected to a primary reverse osmosis treatment, a secondary reverse osmosis treatment, and a tertiary reverse osmosis treatment to obtain purified water, and the residual liquid phase after the primary reverse osmosis treatment is subjected to a reverse osmosis treatment to obtain a concentrate.

[0034] The concentrate described in this article is used to leaching the primary filter residue, which can not only extract the lithium ions contained in the concentrate, but also efficiently leach the lithium ions in the filter residue, ultimately improving the overall lithium recovery rate.

[0035] Compared with the existing technology, the beneficial effects of this article are:

[0036] This article provides a method for recycling and utilizing waste lithium iron phosphate. The method specifically reprocesses the precipitate and leachate obtained by separating and leaching waste lithium iron phosphate powder, and uses membrane concentration to highly enrich the lithium ions in the leachate and fully utilizes the filtrate generated during the treatment process. Ultimately, lithium salts and iron phosphate products can be efficiently recovered and obtained at the same time. The overall method can achieve a lithium recovery rate of more than 95% for waste lithium iron phosphate. At the same time, the concentration of impurity elements in the obtained iron phosphate does not exceed 5ppm, and the quality of the iron phosphate product is high. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] FIG1 is a schematic flow chart of the method for recycling waste lithium iron phosphate described in this article. DETAILED DESCRIPTION

[0038] To better illustrate the purpose, technical solutions and advantages of this article, this article will be further described below with reference to the accompanying drawings and specific embodiments.

[0039] Unless otherwise specified, the materials used in the examples and comparative examples can be obtained through commercial channels.

[0040] The electrodes containing waste lithium iron phosphate used in the various embodiments and comparative examples herein are all positive electrodes in commercially recycled lithium iron phosphate batteries. The main components are waste lithium iron phosphate, conductive agent and binder. The content of lithium, phosphorus and iron elements in the electrodes are tested before recycling.

[0041] The membrane concentration system described in this article consists of a nanofiltration membrane system with a pore size of 1 to 10 nm and a reverse osmosis membrane system with a pore size of 0.4 to 0.6 nm. The rate at which the primary filtrate enters the membrane concentration system is 400 L / h, the pH is 2 to 4, and the pressure is 1 to 1.5 MPa. After the primary filtrate is concentrated by the membrane system, the lithium ion concentration in the solution is 20 to 25 g / L.

[0042] In each embodiment described herein, the pH of the secondary filtrate and the tertiary filtrate is adjusted to 9-10, filtered, and then the pH is adjusted to 4-6 and filtered again; the resulting filtrate is treated using a reclaimed water system, the reclaimed water system including a fine filtration-ultrafiltration system and a reverse osmosis system, the fine filtration-ultrafiltration system including a microporous precision filter, a plate heat exchanger, an ultrafiltration device, and an ultrafiltration water production tank connected in sequence; the reverse osmosis system includes a first-stage reverse osmosis device, a second-stage reverse osmosis device, a third-stage reverse osmosis device and corresponding connected water tanks connected in sequence, and the third-stage reverse osmosis device is connected to a terminal reverse osmosis device.

[0043] After the filtrate flows into the reclaimed water system, it passes through the microporous precision filter and plate heat exchanger of the fine filtration-ultrafiltration system in sequence, and the ultrafiltration device completes the fine filtration and ultrafiltration treatment, and accumulates water in the ultrafiltration water production tank. The water then passes through the reverse osmosis treatment of the first-stage reverse osmosis device, the second-stage reverse osmosis device, and the third-stage reverse osmosis device to obtain purified water. The purified water is recycled, and the liquid in the concentrated water tank of the first-stage reverse osmosis device is collected and passed through the terminal reverse osmosis device to obtain a concentrate.

[0044] Example 1

[0045] An embodiment of the method for recycling waste lithium iron phosphate described herein, as shown in FIG1 , comprises the following steps:

[0046] (1) 100 g of electrode powder containing waste lithium iron phosphate was placed in 1000 mL of 1 mol / L sulfuric acid solution, and 30 mL of 30% hydrogen peroxide was pumped into the solution using a peristaltic pump for leaching for 4 h. After the selective lithium extraction step was completed, a leachate and a leach residue were obtained;

[0047] (2) The leachate is adjusted to pH 8.5 with a saturated calcium hydroxide solution, and subjected to precipitation reaction at 50°C for 2 hours. The precipitate is filtered out to obtain a filtrate and a filter residue, completing one-step impurity removal;

[0048] (3) After the primary filtrate is concentrated by a membrane system, 60 g of sodium carbonate as a precipitant is added to the obtained highly concentrated lithium solution and precipitated at 90° C. for 3 h to obtain lithium salt lithium carbonate and a secondary filtrate;

[0049] (4) Under a nitrogen atmosphere, the leached residue obtained in step (1) was placed in a reactor containing 500 mL of 1 mol / L sulfuric acid solution, and after reacting for 1 hour, the reducing agent elemental iron powder and the additive titanium dioxide were added in stages and mixed, and the mixture was reacted at 50° C. for 3 hours and the insoluble residue was filtered out to achieve acid dissolution and impurity removal. The resulting mixed solution was transferred to another reactor and 1.2 MPa of oxygen was added to react at 80° C. for 6 hours, and filtered to obtain iron phosphate and three filtrates; the mass ratio of the leached residue, elemental iron powder and titanium dioxide was 100:31:65;

[0050] (5) The secondary filtrate and the tertiary filtrate are mixed and adjusted to a pH of 9-10 with sodium hydroxide, and then the pH is adjusted to 4-6 and filtered. The pH of the concentrate obtained after entering the reclaimed water system is 3. The concentrated water is placed in the primary filter residue and leached. The liquid-solid ratio during leaching is 3 mL / g. After filtration, the fourth filtrate and the final mixed residue that cannot be further extracted with lithium are obtained. The fourth filtrate is treated according to the treatment steps of the primary filtrate.

[0051] Example 2

[0052] An embodiment of the method for recycling waste lithium iron phosphate described herein includes the following steps:

[0053] (1) 200 g of electrode powder containing waste lithium iron phosphate was placed in 1000 mL of 1 mol / L sulfuric acid solution, and 60 mL of 30% hydrogen peroxide was pumped into the solution using a peristaltic pump for leaching for 4 h. After the selective lithium extraction step was completed, a leachate and a leach residue were obtained;

[0054] (2) The leachate is adjusted to pH 8.5 with a 30% sodium hydroxide solution, and subjected to precipitation reaction at 50° C. for 2 h. The precipitate is filtered out to obtain a primary filtrate and a primary filter residue, completing one-step impurity removal;

[0055] (3) After the primary filtrate is concentrated by a membrane system, 276 g of sodium phosphate dodecahydrate as a precipitant is added to the obtained highly concentrated lithium solution and precipitated at 90° C. for 3 h to obtain lithium salt lithium phosphate and a secondary filtrate;

[0056] (4) Under a nitrogen atmosphere, the leached residue obtained in step (1) is placed in a reactor containing 500 mL of 1 mol / L sulfuric acid solution, and after reacting for 2 hours, the reducing agent elemental iron powder and the additives titanium dioxide and titanium hydrogen phosphate monohydrate are added in stages, mixed, reacted at 80° C. for 5 hours, and the insoluble residue is filtered out to achieve acid dissolution and impurity removal. The resulting mixed solution is transferred to another reactor and 1.3 MPa of oxygen is added to react at 80° C. for 6 hours, filtered, and iron phosphate and three filtrates are obtained; the mass ratio of the leached residue, elemental iron powder, titanium dioxide and titanium hydrogen phosphate monohydrate is 100:36:80:80;

[0057] (5) The secondary filtrate and the tertiary filtrate were treated in the same manner as in Example 1 to obtain a concentrate, and the concentrated water of the concentrate was placed in the primary filter residue and leached, with a liquid-to-solid ratio of 4 mL / g during leaching. The concentrate was filtered to obtain a fourth filtrate and a mixed residue from which lithium could not be further extracted. The fourth filtrate was treated in the same manner as the primary filtrate treatment steps.

[0058] Example 3

[0059] An embodiment of the method for recycling waste lithium iron phosphate described herein includes the following steps:

[0060] (1) 200 g of electrode powder containing waste lithium iron phosphate was placed in 1000 mL of 1 mol / L sulfuric acid solution, and 60 mL of 30% hydrogen peroxide was pumped into the solution using a peristaltic pump for leaching for 4 h. After the selective lithium extraction step was completed, a leachate and a leach residue were obtained;

[0061] (2) The leachate is adjusted to pH 8.5 with a saturated calcium hydroxide solution, and subjected to precipitation reaction at 60°C for 2 hours. The precipitate is filtered out to obtain a filtrate and a filter residue, completing one-step impurity removal;

[0062] (3) After the primary filtrate is concentrated by a membrane system, 84 g of sodium carbonate as a precipitant is added to the obtained highly concentrated lithium solution and precipitated at 90° C. for 3 h to obtain lithium salt lithium carbonate and a secondary filtrate;

[0063] (4) Under a nitrogen atmosphere, the leached residue obtained in step (1) is placed in a reactor containing 500 mL of 1 mol / L sulfuric acid solution, and after reacting for 3 hours, the reducing agent elemental iron powder and the additive titanium hydrogen phosphate monohydrate are added in stages, mixed, reacted at 50° C. for 6 hours, and the insoluble residue is filtered out to achieve acid dissolution and impurity removal. The resulting mixed solution is transferred to another reactor and 1.3 MPa of oxygen is added to react at 80° C. for 6 hours, filtered, and iron phosphate and three filtrates are obtained; the mass ratio of the leached residue, elemental iron powder and titanium hydrogen phosphate monohydrate is 100:41:100;

[0064] (5) The secondary filtrate and the tertiary filtrate were treated in the same manner as in Example 1 to obtain a concentrate, and the concentrated water of the concentrate was placed in the primary filter residue and leached, with a liquid-to-solid ratio of 5 mL / g during leaching. The concentrate was filtered to obtain a fourth filtrate and a mixed residue from which lithium could not be further extracted. The fourth filtrate was treated in the same manner as the primary filtrate treatment steps.

[0065] Example 4

[0066] An embodiment of the method for recycling waste lithium iron phosphate described herein includes the following steps:

[0067] (1) 200 g of electrode powder containing waste lithium iron phosphate was placed in 1000 mL of 1 mol / L sulfuric acid solution, and 60 mL of 30% hydrogen peroxide was pumped into the solution using a peristaltic pump for leaching for 4 h. After the selective lithium extraction step was completed, a leachate and a leach residue were obtained;

[0068] (2) The leachate is adjusted to pH 8.5 with a saturated calcium hydroxide solution, and subjected to precipitation reaction at 50°C for 2 hours. The precipitate is filtered out to obtain a filtrate and a filter residue, completing one-step impurity removal;

[0069] (3) After the primary filtrate is concentrated by a membrane system, 96 g of sodium carbonate as a precipitant is added to the obtained highly concentrated lithium solution and precipitated at 90° C. for 3 h to obtain lithium salt lithium carbonate and a secondary filtrate;

[0070] (4) Under a nitrogen atmosphere, the leached residue obtained in step (1) is placed in a reactor containing 500 mL of a 1 mol / L sulfuric acid solution, and then the reducing agent elemental iron powder and the additive mixture of dihydrated ferric phosphate and titanium oxide are added in stages and mixed. After reacting for 2 hours, the mixture is reacted at 50° C. for 5 hours and the insoluble residue is filtered out to achieve acid dissolution and impurity removal. The resulting mixed solution is transferred to another reactor and 1.5 MPa of oxygen is added to react at 80° C. for 6 hours, filtered, and ferric phosphate and three filtrates are obtained; the mass ratio of the leached residue, elemental iron powder, and the mixture of dihydrated ferric phosphate and titanium oxide is 100:32:85;

[0071] (5) The secondary filtrate and the tertiary filtrate were treated in the same manner as in Example 1 to obtain a concentrate, and the concentrated water of the concentrate was placed in the primary filter residue and leached, with a liquid-to-solid ratio of 7 mL / g during leaching. The concentrate was filtered to obtain a fourth filtrate and a mixed residue from which lithium could not be further extracted. The fourth filtrate was treated in the same manner as the primary filtrate treatment steps.

[0072] Example 5

[0073] An embodiment of the method for recycling waste lithium iron phosphate described herein includes the following steps:

[0074] (1) 200 g of electrode powder containing waste lithium iron phosphate was placed in 1000 mL of 1 mol / L sulfuric acid solution, and 60 mL of 30% hydrogen peroxide was pumped into the solution using a peristaltic pump for leaching for 4 h. After the selective lithium extraction step was completed, a leachate and a leach residue were obtained;

[0075] (2) The leachate is adjusted to pH 8.5 with a saturated calcium hydroxide solution, and subjected to precipitation reaction at 50°C for 2 hours. The precipitate is filtered out to obtain a filtrate and a filter residue, completing one-step impurity removal;

[0076] (3) After the primary filtrate is concentrated by a membrane system, 75 g of sodium carbonate as a precipitant is added to the obtained highly concentrated lithium solution and precipitated at 90° C. for 3 h to obtain lithium salt lithium carbonate and a secondary filtrate;

[0077] (4) Under a nitrogen atmosphere, the leached residue obtained in step (1) is placed in a reactor containing 500 mL of 1 mol / L sulfuric acid solution, and after reacting for 2 hours, the reducing agent elemental iron powder and the additive ferrous phosphate octahydrate are added in stages, the mixture is reacted at 50° C. for 5 hours, and the insoluble residue is filtered out to achieve acid dissolution and impurity removal. The resulting mixed solution is transferred to another reactor and hydrogen peroxide with a concentration of 30% is continuously added at a rate of 0.7 mL / min, and the mixture is reacted at 80° C. for 6 hours. The mixture is filtered to obtain ferric phosphate and three filtrates; the mass ratio of the leached residue, elemental iron powder and ferrous phosphate octahydrate is 100:52:100;

[0078] (5) The secondary filtrate and the tertiary filtrate were treated in the same manner as in Example 1 to obtain a concentrate, and the concentrated water of the concentrate was placed in the primary filter residue and leached, with a liquid-to-solid ratio of 6 mL / g during leaching. The concentrate was filtered to obtain a fourth filtrate and a mixed residue from which lithium could not be further extracted. The fourth filtrate was treated in the same manner as the primary filtrate treatment steps.

[0079] Example 6

[0080] An embodiment of the method for recycling waste lithium iron phosphate described herein includes the following steps:

[0081] (1) 200 g of electrode powder containing waste lithium iron phosphate was placed in 1000 mL of 1 mol / L sulfuric acid solution, and 60 mL of 30% hydrogen peroxide was pumped into the solution using a peristaltic pump for leaching for 4 h. After the selective lithium extraction step was completed, a leachate and a leach residue were obtained;

[0082] (2) The leachate is adjusted to pH 8.5 with a saturated calcium hydroxide solution, and subjected to precipitation reaction at 50°C for 2 hours. The precipitate is filtered out to obtain a filtrate and a filter residue, completing one-step impurity removal;

[0083] (3) After the primary filtrate is concentrated by a membrane system, 75 g of sodium carbonate as a precipitant is added to the obtained highly concentrated lithium solution and precipitated at 90° C. for 3 h to obtain lithium salt lithium carbonate and a secondary filtrate;

[0084] (4) Under a nitrogen atmosphere, the leached residue obtained in step (1) was placed in a reactor containing 500 mL of 1 mol / L sulfuric acid solution, and after reacting for 2 h, the reducing agent elemental iron powder and the additive ferric phosphate dihydrate were added in stages, and the mixture was reacted at 50° C. for 5 h and the insoluble residue was filtered out to achieve acid dissolution and impurity removal. The resulting mixed solution was transferred to another reactor and 30% hydrogen peroxide was continuously added at a rate of 1.1 mL / min, and the reaction was carried out at 80° C. for 6 h. After filtering, ferric phosphate and three filtrates were obtained; the mass ratio of the leached residue, elemental iron powder and ferric phosphate dihydrate was 100:40:90;

[0085] (5) The secondary filtrate and the tertiary filtrate were treated in the same manner as in Example 1 to obtain a concentrate, and the concentrated water of the concentrate was placed in the primary filter residue and leached, with a liquid-to-solid ratio of 6 mL / g during leaching. The concentrate was filtered to obtain a fourth filtrate and a mixed residue from which lithium could not be further extracted. The fourth filtrate was treated in the same manner as the primary filtrate treatment steps.

[0086] Example 7

[0087] An embodiment of the method for recycling waste lithium iron phosphate described herein includes the following steps:

[0088] (1) 200 g of electrode powder containing waste lithium iron phosphate was placed in 1000 mL of 1 mol / L sulfuric acid solution, and 60 mL of 30% hydrogen peroxide was pumped into the solution using a peristaltic pump for leaching for 4 h. After the selective lithium extraction step was completed, a leachate and a leach residue were obtained;

[0089] (2) The leachate is adjusted to pH 8.5 with a saturated calcium hydroxide solution, and subjected to precipitation reaction at 50°C for 2 hours. The precipitate is filtered out to obtain a filtrate and a filter residue, completing one-step impurity removal;

[0090] (3) After the primary filtrate is concentrated by a membrane system, 75 g of sodium carbonate as a precipitant is added to the obtained highly concentrated lithium solution and precipitated at 90° C. for 3 h to obtain lithium salt lithium carbonate and a secondary filtrate;

[0091] (4) Under a nitrogen atmosphere, the leached residue obtained in step (1) was placed in a reactor containing 500 mL of 1 mol / L sulfuric acid solution, and after reacting for 2 h, the reducing agent elemental iron powder and the additive aluminum hydroxide were added in stages and mixed, and the mixture was reacted at 50° C. for 5 h and the insoluble residue was filtered out to achieve acid dissolution and impurity removal. The obtained mixed solution was transferred to another reactor and 30% hydrogen peroxide was continuously added at a rate of 0.8 mL / min, and the mixture was reacted at 80° C. for 6 h. After filtering, ferric phosphate and three filtrates were obtained; the mass ratio of the leached residue, elemental iron powder and aluminum hydroxide was 100:62:80;

[0092] (5) The secondary filtrate and the tertiary filtrate were treated in the same manner as in Example 1 to obtain a concentrate, and the concentrated water of the concentrate was placed in the primary filter residue and leached, with a liquid-to-solid ratio of 6 mL / g during leaching. The concentrate was filtered to obtain a fourth filtrate and a mixed residue from which lithium could not be further extracted. The fourth filtrate was treated in the same manner as the primary filtrate treatment steps.

[0093] Example 8

[0094] An embodiment of the method for recycling waste lithium iron phosphate described herein includes the following steps:

[0095] (1) 200 g of electrode powder containing waste lithium iron phosphate was placed in 1000 mL of 1 mol / L sulfuric acid solution, and 60 mL of 30% hydrogen peroxide was pumped into the solution using a peristaltic pump for leaching for 4 h. After the selective lithium extraction step was completed, a leachate and a leach residue were obtained;

[0096] (2) The leachate is adjusted to pH 8.5 with a saturated calcium hydroxide solution, and subjected to precipitation reaction at 50°C for 2 hours. The precipitate is filtered out to obtain a filtrate and a filter residue, completing one-step impurity removal;

[0097] (3) After the primary filtrate is concentrated by a membrane system, 75 g of sodium carbonate as a precipitant is added to the obtained highly concentrated lithium solution and precipitated at 90° C. for 3 h to obtain lithium salt lithium carbonate and a secondary filtrate;

[0098] (4) Under a nitrogen atmosphere, the leached residue obtained in step (1) is placed in a reactor containing 500 mL of 1 mol / L sulfuric acid solution, and after reacting for 1 hour, the reducing agent elemental iron powder and additives titanium trioxide and ferrous phosphate octahydrate are added in stages, the mixture is reacted at 50° C. for 4 hours, and the insoluble residue is filtered out to achieve acid dissolution and impurity removal. The resulting mixed solution is transferred to another reactor and hydrogen peroxide with a concentration of 30% is continuously added at a rate of 1.5 mL / min, and the reaction is carried out at 80° C. for 6 hours. The mixture is filtered to obtain ferric phosphate and three filtrates; the mass ratio of the leached residue, elemental iron powder, titanium trioxide, and ferrous phosphate octahydrate is 100:62:80:80;

[0099] (5) The secondary filtrate and the tertiary filtrate were treated in the same manner as in Example 1 to obtain a concentrate, and the concentrated water of the concentrate was placed in the primary filter residue and leached, with a liquid-to-solid ratio of 6 mL / g during leaching. The concentrate was filtered to obtain a fourth filtrate and a mixed residue from which lithium could not be further extracted. The fourth filtrate was treated in the same manner as the primary filtrate treatment steps.

[0100] Comparative Example 1

[0101] A method for recycling waste lithium iron phosphate, which differs from Example 3 only in that the primary filter residue in step (5) is directly leached with pure water, the liquid-to-solid ratio during leaching is 5 mL / g, and filtration is performed to obtain four filtrates and mixed residues, and the four filtrates are treated according to the primary filtrate treatment steps.

[0102] Comparative Example 2

[0103] A method for recycling waste lithium iron phosphate, which differs from Example 3 only in that the nitrogen atmosphere in step (4) is replaced by an air atmosphere.

[0104] Comparative Example 3

[0105] A method for recycling waste lithium iron phosphate, which differs from Example 3 only in that no additive is added in step (4).

[0106] Comparative Example 4

[0107] A method for recycling waste lithium iron phosphate, which differs from Example 7 only in that the mass ratio of the leached residue, elemental iron powder and aluminum hydroxide is 100:62:300.

[0108] Comparative Example 5

[0109] A method for recycling waste lithium iron phosphate, which differs from Example 5 only in that the mass ratio of the leached residue, elemental iron powder and ferrous phosphate octahydrate is 100:52:400.

[0110] Comparative Example 6

[0111] A method for recycling waste lithium iron phosphate, which differs from Example 5 only in that the mass ratio of the leached residue, elemental iron powder and ferrous phosphate octahydrate is 100:52:20.

[0112] Comparative Example 7

[0113] A method for recycling waste lithium iron phosphate, which differs from Example 4 only in that the primary filtrate in step (3) is concentrated using a commercially available MVR evaporation system.

[0114] Comparative Example 8

[0115] A method for recycling waste lithium iron phosphate, which differs from Example 8 only in that the pH of the leachate is adjusted to 3 in step (2).

[0116] Comparative Example 9

[0117] A method for recycling waste lithium iron phosphate, which differs from Example 8 only in that the pH of the leachate is adjusted to 12 in step (2).

[0118] Comparative Example 10

[0119] A method for recycling waste lithium iron phosphate, which differs from Example 8 only in that the pH of the concentrate in step (5) is 1.

[0120] Comparative Example 11

[0121] A method for recycling waste lithium iron phosphate, which differs from Example 8 only in that the pH of the concentrate in step (5) is 6.

[0122] Comparative Example 12

[0123] A method for recycling waste lithium iron phosphate, which differs from Example 8 only in that the concentrate obtained in step (5) is not subjected to fine filtration or ultrafiltration, and the total concentration of impurity elements other than lithium is 0.08 g / L.

[0124] Effect Example 1

[0125] In order to verify the efficiency of the method for recycling waste lithium iron phosphate described in this article, the impurity content of the iron phosphate prepared by the method described in each embodiment and comparative example, the iron and phosphorus content of the iron phosphate product and the molar content ratio of the two, as well as the specific surface area of ​​the iron phosphate product were detected and counted. Finally, the lithium recovery rate of the method was calculated. The lithium recovery rate was calculated as 100-m (lithium in the mixed slag) / m (lithium in the lithium iron phosphate electrode powder); if there is no reclaimed water reuse system, only pure water is used to wash the lithium-containing mixed slag. The lithium recovery rate is calculated as 100-m (lithium in the mixed slag, lithium in the ferrophosphorus slag, lithium in the lithium precipitation mother liquor) / m (lithium in the lithium iron phosphate electrode powder), and finally the purity of the prepared lithium salt is tested.

[0126] The test results are shown in Table 1, Table 2 and Table 3.

[0127] Table 1

[0128] Table 2

[0129] Table 3

[0130] It can be clearly seen from Tables 1 and 2 that after the implementation of the waste lithium iron phosphate recovery method described in this article, the lithium recovery rate can reach more than 96%, and the prepared iron phosphate product has high purity, the main impurity concentration does not exceed 5ppm, and the specific surface area is moderate, and the phosphorus to iron ratio corresponds to that of the standard product. In contrast, the method described in Comparative Example 1 does not recover the secondary filtrate and the tertiary filtrate and leaches the primary filter residue, so the lithium ions in the waste lithium iron phosphate cannot be fully recovered, and the lithium recovery rate is low; in the method described in Comparative Example 2, the conversion process of the leached residue is not carried out under a nitrogen atmosphere, and dissolution in an air atmosphere will cause some iron and phosphorus to be lost, and the iron and phosphorus content of the prepared product is low, and the specific surface area of ​​the prepared product is low; in the product of Comparative Example 3, no additives are introduced for synergistic effect during the conversion of the leached residue, so a good impurity removal and precipitation effect cannot be achieved, and the concentration of titanium impurities is high, resulting in a relatively high titanium content in the final prepared product, and the yield of the iron phosphate product is low; however, the content of aluminum hydroxide, an additive introduced in the product of Comparative Example 4, is too high, so that the aluminum element of the final product is high, indicating that the amount of additive added in the method cannot be too much, and similarly, the amount of ferrous phosphate octahydrate added to the product of Comparative Example 5 is too much, and the quality and specific surface area of ​​the iron phosphate are not ideal; the amount of additive added in Comparative Example 6 is too little, and similar to Comparative Example 3, the titanium impurities cannot be effectively removed. Comparative Example 7 employed a relatively conventional MVR evaporation system to evaporate and concentrate the primary filtrate. This method is more susceptible to lithium ion loss than membrane concentration processes. Comparative Examples 8 and 9 adjusted the leachate pH to excessively high or low levels, resulting in excessively high impurity concentrations or low lithium recovery rates in the prepared lithium salt products. Furthermore, in Comparative Examples 10 and 11, the high or low pH of the concentrate during leaching of the primary filter residue resulted in low lithium ion recovery rates or high impurity levels in the resulting lithium salt. In Comparative Example 12, the impurity content of the concentrate itself was too high, resulting in a high impurity content in the final lithium salt.

[0131] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this article and are not intended to limit the scope of protection of this article. Although this application has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of this article may be modified or replaced by equivalents without departing from the essence and scope of the technical solutions of this article.

Claims

1. A method for recycling waste lithium iron phosphate, characterized in that: The following steps are involved: Placing the pole piece containing waste lithium iron phosphate into an acid solution containing a first oxidant for leaching to obtain a leaching solution and leaching residue; The leachate is adjusted to pH 8-9, impurities are precipitated and filtered to obtain a primary filtrate and a primary filter residue; The primary filtrate is concentrated by a membrane system, and then a precipitant is added to precipitate to obtain lithium salt and a secondary filtrate; The leached residue is reacted with an acid solution under an inert atmosphere, and then a reducing agent and an additive are added for mixed reaction, filtered, and the obtained mixed solution is added with a second oxidant for reaction, filtered, and iron phosphate and a tertiary filtrate are obtained; the reducing agent is elemental iron, and the additive is at least one of ferric phosphate dihydrate, ferrous phosphate octahydrate, titanium hydrogen phosphate monohydrate, aluminum hydroxide, titanium dioxide, and titanium trioxide; the mass ratio of the leached residue, the reducing agent and the additive is 100: (20-65): (50-200); The secondary filtrate and the tertiary filtrate are mixed and the pH is adjusted to 9-10, filtered, and then the pH is adjusted to 4-6 and filtered again; the obtained filtrate is treated by fine filtration, ultrafiltration and reverse osmosis to obtain purified water and a concentrate with a pH of 2-4, the concentrate is placed in a primary filter residue and leached, filtered, and a fourth filtrate is obtained, which is treated according to the primary filtrate treatment steps.

2. The method for recycling waste lithium iron phosphate according to claim 1, characterized in that: The first oxidant is at least one of oxygen, hydrogen peroxide, and sodium persulfate; the acid solution is an aqueous solution of at least one of sulfuric acid and hydrochloric acid.

3. The method for recycling waste lithium iron phosphate according to claim 1, characterized in that: The membrane system comprises a nanofiltration membrane system and a reverse osmosis membrane system in sequence.

4. The method for recycling waste lithium iron phosphate according to claim 1, characterized in that: The precipitant is at least one of sodium carbonate and sodium phosphate dodecahydrate, the temperature during the precipitation is 80-100° C., and the time is 2-4 hours.

5. The method for recycling waste lithium iron phosphate according to claim 1, characterized in that: The reaction time of the leached residue with the reducing agent and the additive is 3 to 6 hours.

6. The method for recycling waste lithium iron phosphate according to claim 1, characterized in that: The second oxidant is one of oxygen and hydrogen peroxide.

7. The method for recycling waste lithium iron phosphate as claimed in claim 6, characterized in that: When the second oxidant is hydrogen peroxide, the addition rate of the hydrogen peroxide is 0.7-1.1 mL / min.

8. The method for recycling waste lithium iron phosphate as claimed in claim 6, characterized in that: When the second oxidant is oxygen, the pressure of the introduced oxygen is 1.2-1.5 MPa.

9. The method for recycling waste lithium iron phosphate according to claim 1, characterized in that: The temperature of the mixed solution when reacting with the second oxidant is ≥80°C.

10. The method for recycling waste lithium iron phosphate according to claim 1, characterized in that: The unit dosage of the concentrate when leaching the filter residue once is 3-7 mL / g.

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